I have a bunch of obvious nominations to post, and some that should also be considered as well as opinion on the other noms. Let's get to the roll:
A- -> A
I know it had recently risen, but there are several questions that must be thought before saying anything. Is there any team in the actual metagame where Trapinch doesn't fit? Is there any situation in which this Pokemon hadn't done anything for its team, be it removing Pokémon such as Onix, Pawniard and Alolan Grimer, or even offensive Pokémon such as Abra and Ponyta? Trapinch usage is still skyrocketing and shows no sign of stopping, and a higher rank should be given to Trapinch to reflect its dominance and presence, letting Gastly + Abra or Ponyta to go rampant over many metagame common builds. It can even countertrap Scarf Diglett, used to deal with Abra, being a big thing already for the core. However, it's not all it can do for its teammates, even other less viable offensive cores such as Flying spam and other less viable Pokemon such as Weak Armor Waterium Z Omanyte benefit from removing Tirtouga/Onix or Pawniard, to put an example in both cases. Rise it up.
A -> A-
Keeping it short and steady, Magnemite is not enjoying this metagame state. Trapinch and Diglett are everywhere, and on top of that, it can not act as the Steel-type trapper it is if there is any of them in the opposing team. This relegates Magnemite to use its best set, Analytic, which still packs quite a big punch to switch-ins and abuses its typing, but much harder with Trapinch being able to swap its bulk. Shoot it down.
A -> A-
Yet another victim of the metagame state, Diglett and Trapinch are everywhere, and Gastly + Abra pummel Mareanie down to oblivion. Its typing can't be fully utilized, as the momentum loss it provokes makes it unable to use its utility options, just to get crippled and pivoted on to be bait for Diglett and Trapinch. To add salt on the bruise, even Pokemon that used to check such as Spritzee are running Psychic due to it being a better filler than Covet (finds no room in this state), and Foongus keeps being an nuisance that outclasses Mareania as a bulky Regenerator user that checks Fighting-types due to its better typing and Spore. Iron Defense is right now what it can make use of to avoid getting easily trapped, but as long as the offensive core is healthy, Mareanie is doomed. Down with it.
A- -> B+
Please, take this thing down already. It's just painful to see how the best not-niche counter to GastBra is bodied to death with the prominence of Trapinch and Diglett, mostly getting easily trapped by Trapinch with only a switch. Definitely not the best moment Alolan Grimer is experimenting, and should go down soon.
A -> A+ Agree
Offensive sets on it are great as Trapinch gets rid of both Onix and Tirtouga, both solid checks to non-Grass coverage Ponyta. Firium Z High Horsepower is real strong right now, Fire coverage alone is enough to mess with the whole metagame and burn it to ashes, which eases Ponyta work at sweeping. However, the best set that takes advantage of Trapinch's abilities is bulky utility. After its checks have been removed, Ponyta starts spreading burns easily, and Flame Charge avoids it from being trapped by Diglett, and can't be trapped by Trapinch if Ponyta burns it. Definitely one of the Pokemon worth using on a team of these characteristics, rise it up.
A -> A+ Agree
Said enough about it, covered really well by a wide range of LC users, important piece of the offensive core that is overtaking the whole metagame as of right now. Rise it.
A+ -> S Disagree
While I like tcr's perspective about Abra, this is overvaluating it and I don't think it's the main reason why metagame common builds of GastBra are stupidly great. The main reason, to put it bluntly, comes from the trappers. Abra wouldn't be able to strange whole teams if there wasn't anything that removed Pawniard or Alolan Grimer, hence why Diglett and Trapinch are seen in an almost similar quantity. What is true though is that Abra complements Gastly by dealing better with Fighting-types, while Gastly nukes away anything that bothers Abra, though both share certain coverage options, in some teams one of them must carry it for bopping Ferroseed, being commonly Gastly. This said, Abra doesn't need to be paired with Gastly, but their ability together to devour the whole metagame with sheer force makes them redeemable and worth of a shared rank among the A+ Pokemon.
B+ -> On the verge, leaning to stay B+
I guess Larvesta has got better with trappers being everywhere, mostly Trapinch, who removes Onix and Tirtouga in exchange for a safe pivoting for it and solid checking to Pawniard, but still doesn't enjoy Stealth Rocks and relies on heavy hazard removal support. Though I think there could be more discussion about it, for now it should stay.
All in all, this metagame state is greatly produced by Trapinch's influence, and I do agree that it's already trying to steal Diglett's spotlight as the best trapper, but it's not absolutely like that. Whatever your team needs removed implies using one trapper or another, meaning that both trappers will get their share at trapping specific Pokemon to help any offensive Pokemon to break.